Fluidized bed and dry-spraying procedures have long been used to apply powders to form thermosetting resinous coatings on metal pipes, panels, reinforcing bars, transformer cases, and the like. Most such coatings are far too hard and brittle to provide electrically insulating coatings on wires, because they tend to crack when subjected to severe stress, either upon being bent or upon expanding and contracting with changes in temperature. The stress can be especially severe when the wire has the rectangular cross-section which is usually specified for windings of the largest wire gauges. The coatings must be resistant to abrasion at the high speeds at which the wires may be wound and to chemicals such as hot transformer oils and automotive fluids to which starter motor windings may be exposed.
It is believed that the first powder to form truly satisfactory electrically insulating coatings on wires is disclosed in U.S. Pat. No. 4,267,300 (Guilbert). That powder comprises a blend of (a) polyglycidyl ether of bisphenol A capped with monobasic aromatic acid and (b) trimellitic anhydride. However, coatings formed of that powder would deteriorate if subjected to prolonged exposure to temperatures much above 140.degree. C. Also, that powder cannot be fused and cured at temperatures significantly higher than 370.degree. C. Higher curing temperatures would allow faster production rates.
The trimellitic anhydride used in the Guilbert patent involves several problems:
(1) toxicity, PA1 (2) high melting point (above 250.degree. C.) and the difficulty of dispersing it homogeneously in the polyglycidyl ether, PA1 (3) moisture pickup, and PA1 (4) marginal shelf life. PA1 4% retained on 200 mesh (74-micrometer openings), PA1 19% retained on 325 mesh (44-micrometer openings), PA1 34% retained on 400 mesh (37-micrometer openings), and gel time at 204.degree. C. was 25 seconds. PA1 Dielectric breakdown: 1100 volts/25 micrometers PA1 Dissipation factor: 11% at 150.degree. C.